The present invention relates to a method of making tools of the type which are capable of withstanding moderate to severe shock during impact, and yet have a high degree of hardness and resultant wear-resistance. Such tools can include, for example, circular saw blades, bandsaw blades, scraper blades, router bits, drill bits, boring bits, milling cutters and so on.
In order to achieve the twin objectives of shock resistance and hardness in the same tool, it has been customary to use a composite welded structure formed from two or more metals. The working part of the tool, that performs the cutting and/or wear-resisting function, is formed from a hard material such as hardened tool steel or cemented carbide particles. The supporting body of the tool, on the other hand, is formed from a relatively softer, but tough and ductile, material such as low-alloy steel. In order to join such materials by welding to form a particular tool, at least three different types of manufacturing methods have been employed.
In one such manufacturing method, a hardened piece of the material which ultimately will form the working part of the tool is forcibly applied against a designated working edge on the supporting body of the tool and the two parts are fused together by welding. In Gunzner, U.S. Pat. No. 4,462,293, for example, hardened cutting tips are formed using a sintering process which fuses together high-speed steel particles. The respective preformed cutting tips are then individually welded, by resistance welding, while being pressed against the leading edge of respective teeth on a saw blade body made of low alloy steel. Another example of this type of method is shown in Anderson U.S. Pat. No. 3,034,378 and Kolb U.S. Pat. No. 3,295,396, where a hardened cemented carbide rod is used, instead of preformed cutting tips, in order to eliminate the need for equipment to individually handle such tips. In Anderson and Kolb, the hardened rod is of tungsten carbide particles cemented together with a cobalt constituent, but other binders may also be used as explained in Owen U.S. Pat. No. 2,833,638. In performing the fusing step, this hardened rod is pressed against the leading edge of a selected tooth and resistance welding fuses the rod and the tooth directly together, after which the rod is severed to leave a hardened end segment of the rod on such edge.
The difficulty with the above-described type of process is that the surface of the hardened cutting tip or rod which is fused with the supporting edge of the tool body presents a nonuniform porosity due to variation in the sizes of the particles which have been sintered or cemented together. Accordingly, heat absorption and pressure between the two parts being welded are non-uniform over the area of the weld, with some localized areas experiencing relatively poor conditions for cross-junction diffusion. Moreover, the hardness of the tip or rod material inhibits any malleable conformation of the tip or rod surface to the edge surface, even under substantial pressure. These factors result in voids and relatively concentrated carbide grain formation along such areas which, in turn, make the junction subject to crack development and fracture under shock induced stress.
In a second type of steel composite tool manufacturing method, the working part is joined to the supporting body by melting all of the working part material to be joined. For example, in Connoy U.S. Pat. No. 3,063,310, the end segment of a wire or rod made of high-speed steel is heated until a molten bead forms at the end segment. This bead or globule drops from the end segment onto a selected preheated edge of the supporting part and attaches to this edge, whereupon the process is repeated to attach a second bead to the next selected edge. Connoy U.S. Pat. No. 3,089,945 shows a somewhat similar approach except that the rod and the edge are moved in opposite directions in order to form a continuous globule along a single edge of the supporting part. Yet another version of this method, shown in Neil U.S. Pat. No. 907,167, uses a multi-step casting procedure that includes casting a high carbon steel to form the working part upon an already cast ingot of low-carbon steel which serves as the supporting part, whereupon the resulting composite ingot is worked to its approximate final proportions by rolling.
This second type of process, however, also fails to produce uniform welds because, for example, of the poorly controlled variations in heating time and temperature that arise under this method. The molten globule or ingot does not reach all areas of the junction at the same instant nor is heat drawn away evenly from all areas of the junction as the globule or ingot cools. Furthermore, in methods which resemble those of Connoy, the globule may not remain heated along the junction for a sufficient time for adequate diffusion to occur across the junction, thereby resulting in a nonuniform or incompletely fused junction susceptible to shock-induced fracture as explained above. Also, the size of the globules that can be formed using this method is severely limited. Moreover, in these methods which involve formation of the working part from a molten material, the amount of subsequent working and forming needed to obtain the final configuration of the working part is excessively time-consuming and expensive for production purposes.
In a third type of steel composite tool manufacturing method, a length of hardened wrought steel serving as the working part is forcibly applied in lengthwise abutment against an elongate flat edge provided on the supporting part, to which edge it is then fused by welding. Thereafter, the fused parts typically are annealed in order to relieve strain in the steel along the weld, and the working part is subjected to a final heat treatment in order to harden the steel of that part. Either before or after hardening, a major portion of the working part is typically removed either by cutting or grinding so as to provide a plurality of working edges on the tool. This third type of tool manufacturing process is disclosed, for example, in Anderson et al. U.S. Pat. No. 3,315,548 where the working part used is drawn wire of hardened high-speed steel, and fusing is accomplished by electron beam welding after one side of the wire has been ground smooth for better contact with the flat edge of the supporting metal. Anderson et al. explains that resistance welding, which is a considerably less expensive welding process, is disclosed in Blum, U.S. Pat. No. 1,535,096 but that numerous attempts to duplicate the Blum method had met with failure due to brittleness and unacceptable warping of the resulting weld. Similarly, Replogle U.S. Pat. No. 2,683,923 observes that resistance welding results in a poorly diffused junction and suggests arc welding as an alternative. Like Blum, Replogle uses a thinly rolled sheet of hardened high-speed steel for the working part. Nystrom et al. U.S. Pat. No. 4,144,777 demonstrates how this third type of method is applied to the manufacture of circular saw blades. Bernstein et al. U.S. Pat. No. 3,034,379, after observing that the problems of poor melting and brittleness at the weld are associated with this third type of method, suggests an alternative method which results in a three-layer or "sandwiched" joint. Here the outside layers, as before, are made of high-speed steel and low-alloy steel, respectively, and the extra inside or "barrier" layer is made of a thin-rolled annealed strip of steel having high hot hardness and low heat conductivity, preferably containing at least 12% chromium such as provided in certain types of stainless steels.
Notwithstanding the respective improvements to Blum's basic method, Anderson, Replogle, and even Bernstein achieve only partial success in providing uniformly fault-free welds and in controlling warping over the length of the weld. Each of these references, for example, discloses the need for an additional straightening step, typically involving heat treatment, to remove the distortion caused by warping along the weld. Bernstein, in particular, calls for three separate heat treating steps which relate to processing of the weld, the support body, and the working part, respectively. The shortcomings of the welds produced by all of these references arise as a result of poorly diffused welds with localized areas of brittleness, which are the same shortcomings that have also been encountered, as already noted, in welds made by the other prior methods. The hardened state of the working part material prior to welding is at least partially responsible for these problems.
A further disadvantage of the third method is that the cutting or grinding which is undertaken in order to provide cutting edges on the working part of the tool results in a high proportion of the material of the working part being lost or wasted. This waste increases as the size of the edges increase, so that only the manufacture of tools that have fine-edged teeth is economically feasible where the working part is to be formed from a relatively expensive material such as high-speed steel.
Accordingly, it is a principal object of the present invention to provide a method for making hardened tools of composite steel structure in which the risk of shock-induced crack development at the welded junction between the working part and the supporting body of the tool is minimized.
A related object of the present invention is to provide a method for consistently creating an optimal welded junction uniformly over the entire weld area between the working part and the supporting part of a composite steel tool to maximize the shock resistance of the junction.
Another related object of the present invention is to provide a method of the above type which is low in cost and which can be implemented with relatively few processing steps.